REVIEW OF IRRADIATION HARDENING AND EMBRITTLEMENT EFFECTS IN REFRACTORY METALS RELEVANT TO NUCLEAR THERMAL PROPULSION APPLICATIONS
Nuclear thermal propulsion (NTP) is advantageous for future crewed interplanetary missions because of its capability for high specific impulse, thrust, large abort windows, and good cargo capacity. A fuel under consideration for use in NTP systems is a ceramic metallic (cermet) consisting of fissile fuel particles, such as uranium dioxide (UO2) or uranium nitride (UN), suspended in a structural refractory metal matrix, such as molybdenum (Mo) or tungsten (W). When structural materials are irradiated at low temperatures (below ~0.35 times the melting point) to low doses (0.001 to 0.1 displacements per atom), irradiation hardening and embrittlement may occur. This phenomenon increases the yield strength of materials, but also causes a decline in ductility and an increase in the ductile to brittle transition temperature (DBTT). During operation, large temperature gradients are present throughout the core, causing regions of the fuel element to operate at relatively low temperatures and receive neutron doses conducive to irradiation hardening. A comprehensive literature review was conducted to determine the effects of low-fluence neutron irradiation of pure and alloyed Mo and W. Irradiation hardening occurs in Mo and W up to 1070 and 1100 K respectively, with significant changes in the mechanical properties even at very low neutron doses. The reviewed literature relevant to NTP applications are summarized, knowledge gaps identified, and implications of mechanical property evolution on fuel performance and operating margins discussed.
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